When I was trying to get from ETH to steem I thought I was doin it wrong too.
Basically blocktrades creates a temp wallet to put (let's say bitcoin) to get it to there you need the address for your litecoin deposit. Once it has verified that address for your lightcoin to end up in. Then comes the "temp" address. copy that address. Then go to the bitcoin wallet that you want to withdrawl from to exchange to litecoin. Hit the withdrawl tab. Most likely it is a - symbol. There is a destination box where you would paste the address you got from block trades.
Then the bitcoin is sent to blocktrades they convert it to litecoin then it is deposited into the litecoin wallet.
Check your email sometimes because they might want a verification for the withdrawl of your bitcoin. I got worried when mine didn't go through then I check my email and "ta da" the. The transfer started.